This invention relates to color correction apparatus for use in color video signal receiving apparatus and, more particularly, to such color correction apparatus which is responsive to a VIR signal and which exhibits an improved response time so as to minimize the time required for an initial color correction operation.
Recently, various color television broadcasts have inserted a reference signal during a predetermined, recurrent interval for the purpose of automatically controlling a color television receiver to reproduce a video picture having optimum color characteristics. This reference signal is inserted into the nineteenth line interval of the vertical blanking interval, and is known as the vertical interval reference (VIR) signal.
The VIR signal which has been adopted in the industry is formed of a reference subcarrier, equal in frequency and phase to the usual burst signal which normally is transmitted in successive line intervals of the color video signal, this reference subcarrier being superposed upon some reference luminance level and being transmitted during a preselected chrominance reference portion of the VIR signal. Following this chrominance reference portion, the VIR signal is provided with a luminance reference portion of predetermined amplitude and duration. A black reference portion of a respectively predetermined amplitude and duration then follows the luminance reference portion. As in the transmission of a normal line interval, the VIR signal also includes horizontal synchronizing pulses and a burst signal.
It is expected that, when a transmitted video signal having the VIR signal is received, the reference information provided by the VIR signal can be used to control the color video signal receiving apparatus such that the reproduced color picture exhibits optimum color characteristics. That is, this VIR signal can be used to control the phase of the generated local oscillating signal which, as is known, demodulates the received chrominance component such that the phase of this local oscillating signal determines the hue or tint of the color picture. Also, the information provided by the VIR signal can be used to control the level of the received chrominance component prior to demodulation thereof, so as to control the color saturation of the reproduced color picture. Therefore, by automatically controlling the phase of the local oscillating signal and the level of the received chrominance component in accordance with optimum standards, such as the NTSC standard, as represented by the VIR signal, a correspondingly optimum color video picture can be reproduced.
The subcarrier included in the chrominance reference portion of the VIR signal is in phase quadrature with the R-Y axis. Hence, if the hue of the reproduced color picture is correct, then, during the chrominance reference portion of the VIR signal, it is expected that the demodulated R-Y color difference signal will be equal to some reference level. VIR-controlled color correction circuitry is designed to detect the R-Y color difference signal during the chrominance reference portion of the VIR signal and to adjust the phase of the local oscillating signal until the R-Y color difference signal is equal to the reference level. Correct hue then will be established.
Also, the luminance to chrominance proportioning of the chrominance reference portion of the VIR signal is such that, at the correct saturation level (i.e., when the level of the chrominance component is correct), one of the color drive signals is zero. In particular, the phase of the subcarrier transmitted during the chrominance reference portion is along the -(B-Y) axis, and the luminance of chrominance ratio of the chrominance reference portion is 2.03. This means that the blue drive signal derived from the demodulated B-Y color difference signal is zero. In the VIR-controlled color correction circuitry, the gain of a chrominance amplifier is controlled as a function of the derived blue drive signal so as to adjust the level of the chrominance component until the blue drive signal is detected as zero, thereby establishing the correct color saturation.
The foregoing VIR-controlled color correction is disclosed in U.S. Pat. No. 3,950,780, issued Apr. 13, 1976. In the color correction system disclosed in this patent, the color difference signal which is used for controlling hue and the color drive signal which is used for controlling saturation are compared to reference levels which are derived from the VIR signal itself. However, because of the particular format of the VIR signal, it is necessary for the color difference signal or the color drive signal to be sampled during one vertical blanking interval and to be compared to the derived reference levels during the next following vertical blanking interval. Consequently, the sampled color difference signal and the sampled color drive signal must be stored for the duration of one frame. As a result of inherent leakage and other distortions in the storage circuitry, and because of other changes which may occur from one frame to the next, inaccuracies are introduced into the color correction arrangement disclosed in U.S. Pat. No. 3,950,780.
An improved VIR-controlled color correction circuit is disclosed in our copending application Ser. No. 839,847 filed Oct. 6, 1977. In this improved color correction circuit, the problem of storing a color difference signal or a color drive signal or a derived reference level throughout a frame interval is avoided. The improved circuit proceeds upon the basis that one of the demodulated color difference signals, that is, the R-Y signal, will have the same amplitude during the chrominance reference portion and the immediately following luminance reference portion. Hence, this color difference signal is sampled during the chrominance reference portion and then during the luminance reference portion, and the samples are compared to each other. Any difference therebetween is caused by an error in the hue of the reproduced video picture, and the resultant difference signal is used to adjust the hue accordingly. Similarly, it is known that the level of a simulated color drive signal, such as the blue drive signal, produced during the chrominance reference portion of the VIR signal will be the same during the immediately following luminance reference portion. Hence, the simulated color drive signal is sampled during the chrominance reference portion and then sampled again during the luminance reference portion, these samples being compared to each other. Any difference between the samples of the simulated color drive signal is due to an error in the saturation of the reproduced video picture, and this signal difference is used to adjust the color saturation level accordingly.
In the color correction circuit disclosed in U.S. Pat. No. 3,950,780, and in our aforedescribed improved color correction circuit, the hue and saturation control signals are produced or updated only during the VIR interval, that is, only during every nineteenth line interval. Consequently, storage circuits must be provided to store these control signals for use during the times between which control signals are updated.
If a received video signal does not include a VIR signal, it is important that the control signals which normally are produced in response to the VIR signal are not applied to the hue and saturation control circuits. Typically, switching devices are controlled by a VIR detector for connecting either manually adjustable devices or the VIR-responsive control signals to the hue and saturation control circuits. However, even if the VIR-responsive control signal generators are not connected to the hue and saturation control circuits, nevertheless, such control signal generators may be responsive to the particular color difference signal and simulated color drive signal which are produced during each nineteenth line interval. As a consequence thereof, the hue and saturation control signal storage circuits may store unpredictable, transient signals. Such stored signals may exhibit maximum or minimum levels. If the video signal receiving apparatus which includes this color correction apparatus suddenly is switched from a broadcast channel which does not include a VIR signal to a broadcast channel which does include a VIR signal, the fact that the signals which are stored in the hue and saturation control signal storage circuits are unpredictable and may have maximum or minimum levels that an extended period of time may be necessary in order for such stored signals to change to the proper control signal levels. Similarly, when the power supply for the video signal reproducing apparatus initially is turned on, the signals which are stored in the hue and saturation control signal circuits as a result of turn-on transients may be unpredictable and may exhibit maximum or minimum values. Here too, an extended period of time is required until such stored transient levels are modified, under the control of the VIR signal, to proper hue and saturation control signal levels. This problem also may occur if a spurious noise signal is present during the nineteenth line interval, that is, when a proper VIR signal is expected. Thus, during the time period that the stored hue and saturation control signals are brought to their proper levels, that is, for a period of about one second, a video picture having degraded color characteristics is displayed. This picture is particularly objectionable to a viewer.
When the stored control signals are at maximum or minimum values, as when the VIR signal is not received, or the power supply is energized, or a spurious noise signal interferes with a VIR signal, the loop gain of the hue and saturation control circuits becomes relatively low. This means that even large changes in the control signals will result in relatively small changes in the hue and saturation. Consequently, the responsiveness of the hue and saturation control circuits, that is, the speeds at which these circuits respond to changes in the respective control signals, is slow.